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CHICAGO (AP) — Ryan Miller was a healthy 4-year-old when he woke up one day in his San Francisco-area home and inexplicably started limping.

The same thing happened several months ago in Springfield, Mass., to 6-year-old Paolo Brandon — a mystery limp that came out of the blue. It sent both boys to the doctor’s office, where the possibilities were alarming.

“Leukemia, bone cancer — it was scary to know what it could have been,” said Ellen Weiss of suburban Chicago. Her son, Max, also developed the condition at 7.

All had toxic synovitis, big words for what is essentially a swollen hip joint. Little discussed in baby guides, the rarely serious condition is among the most common causes of limping in young children.

“It just sounded so bizarre,” said Paolo’s mother, Heather. “No one I knew had ever had this.”

About 100,000 U.S. children are diagnosed with the ailment each year, most from ages 3 to 10. Boys are more commonly affected.

Toxic synovitis probably results from an inflammatory response to a virus. The name comes from the synovial membrane lining the hip joint.

“No one knows exactly why it happens,” said Dr. Russ Horowitz, an emergency-room physician at Children’s Memorial Hospital in Chicago.

The good news is it usually goes away within a week or so without treatment other than rest and over-the-counter pain relievers if needed. That’s why it’s sometimes called transient synovitis.

The bad news is there’s no test to identify the condition. It’s diagnosed only by a process of elimination. X-rays and ultrasound often give a clue, but invasive blood and fluid tests and even hospitalization often are needed to rule out less common but more serious ailments it can mimic.

Gracie Leugers of Cincinnati had it just a few weeks ago, accompanied by intense pain that frightened the usually energetic 4-year-old and her family.

“It was just a freak kind of thing that came on, and we had no idea where it came from,” said her mother, Jackie Leugers.

Any unexplained limp in children — the most common feature of toxic synovitis — is a medical warning sign requiring a doctor’s attention.

One of the first conditions doctors want to rule out is called septic arthritis of the hip, a dangerous bacterial infection that can eat away at the bone and cause disability without prompt antibiotic treatment. It can follow a bacterial infection elsewhere in the body that spreads to the hip.

X-rays and other imaging tests can help differentiate the two. Fluid buildup around the hip joint can occur in both, but it’s usually worse in septic arthritis. Blood tests also can detect signs of infection associated with septic arthritis but not toxic synovitis.